The field of the invention is the activation and reactivation of supported silver catalysts which are useful in the preparation of ethylene oxide by the catalytic vapor phase oxidation of ethylene.
The state of the art of processes for preparing ethylene oxide by the catalytic vapor phase oxidation of ethylene with molecular oxygen in the presence of supported silver catalysts may be ascertained by reference to U.S. Pat. Nos. 4,012,425 and 4,039,561, the disclosures of which are incorporated herein. U.S. Pat. Nos. 3,793,231; 3,962,136 and 4,066,575 disclose the state of the art of silver catalysts for the production of ethylene oxide, the disclosures of which are incorporated herein. The reactivation of silver catalysts is disclosed in U.S. Pat. Nos. 4,051,068 and 4,125,480, the disclosures of which are incorporated herein.
Silver catalysts are used in the preparation of ethylene oxide by oxidizing ethylene with oxygen or with gases containing oxygen. It is further known to react such catalysts with so-called promoters, especially the alkali earth metal compounds, preferably barium compounds and/or alkali metal compounds. In particular, the use of heavy alkali metals, rubidium and/or cesium are known from U.S. Pat. Nos. 3,962,136; 4,066,575; 4,039,561. The promoters ordinarily are deposited on the substrate simultaneously with the silver or subsequent to the silver deposition in the preparation of the catalyst.
It is furthermore known that the silver catalysts lose selectivity in the course of time and that after a few years, catalysts must be replaced. The exchange of a catalyst which has lost some of its performance by a new one is time-consuming and is labor intensive in large-scale industrial plants. Furthermore, the exchange causes a production stoppage and high costs.
It is furthermore known to improve the performance of a silver catalyst or to reactivate a used one as disclosed in U.S. Pat. Nos. 4,125,480 and 4,051,068. This is done for instance in large-scale plants by flooding the catalyst-filled reactor with the solution of the promoter metal solution. Following separation of the excess solvent, the residual solvent is removed by heating the catalyst bed and blowing an inert gas through it. This procedure requires using solvents and again a stoppage in production.